The NOAA Air Gap system is a tool that measures the clearance between the water surface and the bridge.

Air gap measurements are updated every six minutes to account for changes in water level and bridge height, due to bridge traffic, air temperature, and other factors. Data on air gap along with real-time data on water conditions like tides, currents, and winds, help ship captains safely enter and leave U.S. ports. This information is critical for a ship captain to safely navigate a ship under a bridge.

For more information:

Center for Operational Oceanographic Products and Services

Physical Oceanographic Real Time System

Ship Under a Bridge

NOAA's Air Gap Technology Sends USS New York Down the Mississippi River for Sea Trials




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NOAA's navigation response teams, part of the Office of Coast Survey, conduct hydrographic surveys of the ocean floor, monitoring for changes in depth or hazards below the surface of the water that could pose great danger to vessel traffic above.

Six navigation teams are strategically located around the country, ready to collect data needed to inform officials of navigational hazards and to help the U.S. Coast Guard find alternative routes for commercial and military ships. NOAA also uses the survey data to update NOAA's national suite of nautical charts—"roadmaps" for mariners. For more information:
Office of Coast Survey
Navigation Response Teams (Diving Deeper podcast, 5.19.10)
Hydrographic Surveying
NOAA's Navigation Response Teams
Rapid Response for Disasters

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Bathymetric data, which includes information about the depths and shapes of underwater terrain, has a range of uses, including:

Bathymetric surveys provide the data on which nautical charts are based.  These charts guide mariners much as road maps guide motorists, ensuring safe and efficient maritime transportation. Bathymetric maps are becoming increasingly important as scientists seek to learn more about the effects of climate change on the environment. Bathymetric surveys can alert scientists to ongoing and potential beach erosion, landslides, sea-level rise, and subsidence (land sinking). Scientists also need current/updated bathymetric survey data, which is critical to support the creation and development of hydrodynamic models. Bathymetry is also a key element of biological oceanography.  The depth and characteristics of the seabed define the habitat for benthic (bottom-dwelling) organisms, and are fundamental parameters of marine ecosystems.  Scientists increasingly rely on high-resolution bathymetry in their efforts to determine where fish and other sea life will feed, live, and breed. For more information:
Office of Coast Survey
Sea-floor Mapping, NOS Education

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The term “bathymetry” originally referred to the ocean’s depth relative to sea level, although it has come to mean “submarine topography,” or the depths and shapes of underwater terrain.

In the same way that topographic maps represent the three-dimensional features (or relief) of overland terrain, bathymetric maps illustrate the land that lies underwater. Variations in sea-floor relief may be depicted by color and contour lines called depth contours or isobaths.

Bathymetry is the foundation of the science of hydrography, which measures the physical features of a water body.  Hydrography includes not only bathymetry, but also the shape and features of the shoreline; the characteristics of tides, currents, and waves; and the physical and chemical properties of the water itself.

For more information:
Office of Coast Survey
Sea-floor Mapping, NOS Education

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Sonar, short for Sound Navigation and Ranging, is helpful for exploring and mapping the ocean because sound waves travel farther in the water than do radar and light waves.

NOAA scientists primarily use sonar to develop nautical charts, locate underwater hazards to navigation, search for and map objects on the sea floor such as shipwrecks, and map the sea floor itself.

There are two types of sonar—active and passive.

Active sonar transducers* emit an acoustic signal or pulse of sound into the water. If an object is in the path of the sound pulse, the sound bounces off the object and returns an “echo” to the sonar transducer. If the transducer is equipped with the ability to receive signals, it measures the strength of the signal. By determining the time between the emission of the sound pulse and its reception, the transducer can determine the range and orientation of the object.

*A transducer is an electrical device that transforms energy from one form to another. Examples are microphones, antenna, and speakers.

Passive sonar systems are used primarily to detect noise from marine objects (such as submarines or ships) and marine animals like whales. Unlike active sonar, passive sonar does not emit its own signal, which is an advantage for military vessels that do not want to be found or for scientific missions that concentrate on quietly “listening” to the ocean. Rather, it only detects sound waves coming towards it. Passive sonar cannot measure the range of an object unless it is used in conjunction with other passive listening devices. Multiple passive sonar devices may allow for triangulation of a sound source.

For more information:
Surveys and Wrecks, Office of Coast Survey
Understanding Ocean Acoustics, NOAA Ocean Explorer
Sound in the Sea Gallery, NOAA Ocean Explorer
Acoustic Monitoring, NOAA Vents Program
Sonar, NOAA Ocean Explorer
Using Sonar to Detect Sea Turtles, NOS Weekly News

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Hydrography is the science that deals with the measurement and description of the physical features of bodies of water and the land areas that are affected by those bodies of water.

A hydrographic survey may be conducted to support a variety of activities: nautical charting, port and harbor maintenance (dredging), coastal engineering (beach erosion and replenishment studies), coastal zone management, and offshore resource development.

The one data type common to all hydrographic surveys is water depth. Of additional concern to most surveys is the nature of the sea-floor material (i.e., sand, mud, rock) due to its implications for anchoring, dredging, structure construction, pipeline and cable routing, and fisheries habitat.

The primary use of hydrographic surveys is for nautical charting.

For more information:
Office of Coast Survey
Sea-floor Mapping, NOS Education


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The U.S. economy is very dependent on the health of the ocean. Consider the following facts: Through the fishing and boating industry, tourism and recreation, and ocean transport, one out of six jobs in the U.S. is marine-related.

Trade across the ocean contributes over $700 billion annually to the U.S. gross domestic product while employing 13 million Americans. Coastal and marine waters support over 28 million jobs, while providing tourist destinations for 189 million Americans each year. U.S. consumers spend over $55 billion for fishery products annually.

For more information:
NOAA's Coastal and Ocean Resource Economics

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